.pytool/EccCheck: Set PACKAGES_PATH env var in Ecc

When running Ecc on other repositories (e.g.: edk2-platforms with
edk2 as a submodule), edk2 modules are referenced.
E.g.: MdePkg/..
The PACKAGES_PATH env var can be used to reference other directories
containing packages. Set it so that Ecc can find these packages.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>

.pytool/Plugin/EccCheck/EccCheck.py

@@ -67,6 +67,7 @@ class EccCheck(ICiBuildPlugin):
         env = shell_environment.GetEnvironment()
         env.set_shell_var('PYTHONPATH', python_path)
         env.set_shell_var('WORKSPACE', workspace_path)
+        env.set_shell_var('PACKAGES_PATH', os.pathsep.join(Edk2pathObj.PackagePathList))
         self.ECC_PASS = True
         self.ApplyConfig(pkgconfig, workspace_path, basetools_path, packagename)
         modify_dir_list = self.GetModifyDir(packagename)